Auerswald COMpact 4000 Operation Manual page 127

Telephoning
Call Waiting Calls
Disconnecting the Current Call and Accepting the Call Waiting Call
Note:
You can also use this function on analogue telephones, where it can be
performed via the T-Net function key/menu (optional, see manual for the
telephone).
On ISDN and system phones, this function is performed via an existing
function key or via the menu (optional, see manual for the telephone).
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COMpact 4000 - Firmware V6.6A - Installation, Operation and Configuration V03 03/2016